The circulation of drilling fluid for the drilling of a deep well provides several benefits some of which are lubrication of the bit, cooling of the bit, removal of cuttings from the borehole, and retention of formation pressures by the weight of the drilling fluid. The drilling fluid is normally pumped by means of conventional surface equipment downwardly through the drill pipe, continuing downwardly through the drill collars, through small holes in the drill bit, up the annulus between the borehole and the drill collars, and thence up the annulus between the borehole and the drill pipe to the surface for reconditioning so as to be recirculated. The route within the well shall be called the flow path herein. The hydrostatic pressure caused by the drilling fluid increases as depth increases, and since the hydrostatic pressure within the drill string is normally equal to the hydrostatic pressure within the annulus at the same depth, the instant invention is not concerned with hydrostatic pressure. However, the present invention is most concerned with pressure on the drilling fluid caused by conventional pumps mounted at the surface. For purposes herein, the pressure imposed by the pumps on the drilling fluid shall be termed circulating pressure herein. Conventionally, fluid enters the drill string under sufficient pump pressure to cause it to circulate along the flow path at a desired flow rate, progressively losing some of the circulating pressure along the flow path due to friction losses within the drilling string across the drill bit, and up the annulus until it becomes zero when it returns to the surface.
On many occasions, threaded joints of the drill string have not sealed against the drilling fluid, allowing a short circuit of the flow path to form through the wall of the drill string and to at least partially bypass the lower drill string, the drill bit, and the lower annulus. Such a bypass condition is generally termed a "washout". If a washout is not located early, it can defeat the purposes of the drilling fluid as stated above and thereby risk stuck pipe, early bit failure, dangerous fishing jobs, loss of the well, a blowout, pollution, loss of energy, extreme costs, and more. The instant invention provides a method for the early detection and location of a washout so as to allow for repair with minimal problems.
Another problem encountered often is the loss of drilling fluid from the borehole into a leach zone, such a problem being generally called "lost circulation". Lost circulation can risk the same results as given above for a washout. The instant invention provides a method for the early location of a zone of lost circulation so as to allow for remedial action with minimal problems. A zone of lost circulation shall herein be called a leach zone. Crites U.S. Pat. No. 2,290,408 discloses a method of detecting and locating leach zones by recording and comparing the difference in the input and output volumes of drilling fluid to the depth of the borehole. If while drilling at a given depth the input volume of drilling fluid substantially exceeded the output volume, it was assumed that the formation at that depth was a leach zone. As well known to those skilled in the art however, formations further up the borehole may begin to receive drilling fluid due to such factors as a change in drilling fluid characteristics, higher pump pressures, or the loss of mud cake on the borehole wall. Nonetheless, Crites input output volume comparison to be of interest for the detection of but not for the location of leach zones. Such interest is evidenced by the recent article by Mans, Peters and Meador of Exxon Production Research Co., the article being published in the Oil & Gas Journal dated Aug. 20, 1979.
Hayward U.S. Pat. No. 2,528,956 discloses a method of locating washouts by injecting an "indicator fluid" and calculating the depth of the washout as follows: divide the volume of fluid pumped into the drill string between the time of injecting the indicator fluid and the time of the return of some of the indicator fluid to the top of the annulus by twice the average cross-sectional area of the flow path to and from the washout. To use this method accurately, the operator must first caliper the borehole or make an accurate assumption of the diameter and he must already know approximately where the washout is if the borehole diameter varies. Also, if a small washout is to be detected, most if not all the inductor fluid will continue down the drill string, bypassing the washout and thereby render detection at the surface of the first returns of the indicator fluid most difficult indeed.
It is a primary feature of this invention to provide a method to determine the depth of a drill string washout quickly and without interrupting drilling operations.
It is another feature of this invention to provide a method to determine the depth of a leach zone quickly and without interrupting drilling operations.
It is still another feature of this invention to provide methods for locating washouts and leach zones that may be practiced automatically and continuously by means of a computer.
It is another feature of this invention to allow for early remedial action so as to correct for washouts or leach zones.
It is another feature of this invention to reduce the probability of a blowout.
It is another feature of this invention to reduce the probability of environmental pollution.
It is also a feature of this invention to reduce the waste of energy while drilling a well.
It is still another feature of this invention to determine whether a sudden reduction in circulating pressure during drilling is caused by a washout, a reduction in pump efficiency, or lost circulation to a leach zone.
It is also a feature of this invention to reduce the cost of drilling a well and thereby to reduce the cost of energy.